You are not wrong, and that’s one of the things that I personally don’t like about the current way fpm handles dependencies. I preferer to get the required packages locally and then declare the dependencies with the path option.
Now, FPM does enable you to “install” in the proper term Manifest reference — Fortran Package Manager which would then enable you to link against your pre-compiled static library.
Another option, which I haven’t tested, is creating a local registry Settings — Fortran Package Manager